Adding mirror reflection on the picture frame that can see the content inside and the reflection of the environment
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Create a plane

Set primary shader to Diffuse BSDF

Plug Diffuse BSDF into a shader to RGB node and route that node to a ColorRamp

Plug the ColorRamp’s alpha into a Principled BSDF shader’s alpha, and then that into a Material Output surface

Go to Render settings, Film section, and check transparent

In the plane’s Material tab, ensure Blend Mode is set to “Alpha Blend”

Make sure that you are in Rendered View (it does not work in Material Preview) and your Reflection Plane is placed slightly above the mirror surface. You can increase the Distance parameter inside Reflection Plane just to be sure that it covers everything.


Cycles
For the mirror material:

Set metallic to 1

Set roughness to 0